Rename mailman postfix override file

compose.mailman.yml

@@ -1,5 +1,9 @@
 version: "3.6"
 
+# TODO(decentral1se): rename `postfix_override` to `mailman_override` because
+#                     there are more override files now and it is good to be
+#                     more specific
+
 services:
   smtp:
     networks:
@@ -25,4 +29,4 @@ volumes:
 configs:
   postfix_override:
     name: ${STACK_NAME}_postfix_overrides_${POSTFIX_OVERRIDE_VERSION}
-    file: postfix.cf
+    file: postfix.mailman.cf